The structural characteristics of a fibre reinforced laminate are usually governed by the amount, type and orientation of the reinforcement fibres. Typically, the stiffness and strength of fibres can only be taken into account to the extent that loading occurs in the longitudinal fibre direction. Therefore, a traditionally designed laminate assumes that the fibres of the finished laminate will be oriented in the same direction as the direction of the fibres when placed in the mould. However, in some cases wrinkles in the fibre layers may occur as a result of the manufacturing process. In such cases the wrinkled fibres no longer have the desired orientation, and severe overload of the laminate may be the result.
Wrinkles may occur for a number of reasons. The thermal expansion of a laminate during curing may exceed the thermal expansion of the mould, in which case the laminate may come under compressive pressure before the matrix material, typically a thermoplastic or thermosetting material, is cured sufficiently to maintain the fibres in the desired orientation. Uneven structures underneath the laminate or undulations in the surface on which the laminate is built up may also include wrinkles.
Wrinkles in fibre reinforced laminates are typically prevented by a combination of arrangements. The laminate thickness is kept below certain limits in order to minimise exothermal heat generation. Moulds and other surfaces on which the laminate is built up are maintained at a high quality. Curing is carried out at carefully controlled temperature gradients so as to minimise differences in the thermal expansion.
Another method for avoiding wrinkles in the fibre layers consists of including layers of wrinkle-preventing material between the fibre layers. Wrinkle-preventing material is manufactured in such a way that it is stiffer than normal fibre material. When positioned between layers of, for example, fibreglass mats, the fibres in the mats are largely prevented from wrinkling, because the mats are kept flat by the wrinkle-preventing material.
The wrinkle-preventing material can be a pre-cured solid, perforated or mesh-like laminate or a solid, perforated or mesh-like material other than a laminate, for example wood or metal.
In US2005/0048260 A1 a method for fabricating a laminated composite body including a metal foil and a plurality of fibre plies is disclosed. The method includes perforating a sheet of metal foil, stacking the perforated metal foil sheet in the plurality of fibre plies in face to face relation in a predetermined order and orientation, and infusing resin into the stacked sheet and plies so that resin flows through the perforations in the metal foil sheet and intersperses between the plurality of fibre plies to form the laminated composite body. Instead of a perforated metal foil also a plurality of metal foil strips may be used to make laminated fibre metal composite structures.
In WO 2004/071761 A1 a laminate of at least two plates formed from aluminium alloy is disclosed, between which an intermediate layer on the basis of plastic which contains at least two groups of continuous, mutually parallel fibres is situated. The intermediate layer is connected to the metal plates.
In WO 95/20479 a method of manufacturing a composite laminate having a plurality of unidirectional orientated layers, for example layers of matrix material reinforced with unidirectional orientated fibres is disclosed. At least one inner metal layer, for example a metal layer that does not form an outer surface of the laminate, together with the other layers is arranged so as to give a balanced and symmetric laminate.